frontend: stop using keyset in GWTRPCCommunicationProvider

Continue to patch http://gerrit.ovirt.org/#/c/21618/
Move to use entryset instead. Solves a findbug warning.

Change-Id: I49ad8873174ecbe1c215749741598cf6c1e9e22e
Signed-off-by: Alissa Bonas <abonas@redhat.com>